Coolant Temp Gauge Rebuild w/parts list & photos

[r39o]

[NorthOf50]

At the time of the rebuild I only had a pack of new blue led on hand (replaced high beam led) . They are also much brighter than the original red so I also thought might do a better job of catching the drivers eye. Also water is blue so why not have a blue led for coolant!?!

Using my diode test on the MM, new blue led Vf = 2.53, original red led Vf = 1.9

Not a huge difference, my rudimentary electric math indicates a small change in the current, maybe an issue anyway?

Either way, I'll undo the gauge tonight and put the original red back in if it'll be helpful.

I was also able to borrow a gauge from the local VW shop (thanks to Glenn's Small Auto in Courtenay, BC) and installed it in the dash. It operates as expected which confirms that I certainly have a gauge issue.
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'84 1.9WBX Camper (Penny)
'88 2.1WBX Camper (Martin)

[NorthOf50]

Jim,

The coolant temp needle taps directly into the 10v input post from the gauge and terminates at the ground post which is connected to the 14/6 wire / coolant temperature sensor. There is nothing else in the circuit apart from it running in parallel to the rest of the gauge.

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'84 1.9WBX Camper (Penny)
'88 2.1WBX Camper (Martin)

[RawUmber]

[NorthOf50]

[kirtfitzpatrick]

A couple of those connections look slightly suspect. Kinda hard to be sure from the photo though. Could you test continuity or resistance between some of the most suspect pins on the backside and its corresponding track on the frontside? It could be something as simple as a bad connection. That would explain everything.
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@tintopcamper
YouTube

[NorthOf50]

Two things:

1 - Solution / workaround found! I had also ordered a GW Audible Coolant Temp alarm kit, before I found this post on how to rebuild the gauge.

https://www.gowesty.com/product/-/24196/gowesty-audible-coolant-temp-alarm-kit?v=

I installed it tonight so see if, like I assumed, it would work despite or in spite of, the LED not working. It does, and it is very loud. I'd recommend one to everyone as unless glued to the gauges, one may miss a blinking red LED. Anyhow, the audible alarm works and I also tried inserting a LED in line with the speaker. When the speaker sounds, it will make the LED flash too. Therefor if I completely botch all of this and can never get the gauge LED to work I plan on bypassing the gauge LED entirely and wiring a "back door" to it from the GW alarm to allow both to work.

2 - A shortage of time has held me up. I suspect the gauge is not working most likely due to me novice soldering. I'm about to dig in there and try to re-work the board if I have no luck testing the bits as they are currently soldered in place.

Will report back later tonight or this weekend. Jim, you circuit diagram was quite helpful. A minor tweak here or there and it's perfect. I already had that mental picture in my head so am happy I snagged a copy before you removed the photo from the post!


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'84 1.9WBX Camper (Penny)
'88 2.1WBX Camper (Martin)

[crazyvwvanman]

The ground post is the middle one of the three on the gauge.
The temp sensor connects to one of the end posts.
The other end post is 10v power into the gauge.

The ground post isn't used by the gauge needle reading.
The ground post is used by the flashing led.

To see which threaded post is which, you can look at the wiring colors in the GW Alarm photos that have wires going to the posts.
https://www.gowesty.com/images/ec/products/large/large_4aef11a09b6ea36254a4ef8ad8c46848.jpg

Mark

[fxr]

OK, back in the game...

It's tricky sorting out the circuit without the device in front of me and some measuring tools while also relying on verbal descriptions, but here's the latest attempt!

I still don't like the LED having no current limiting resistor except for the internal 50ohm resistor apparently inside the 741.

I think I can see how it works apart from that. With the engine cold, the + i/p of the 741 is held higher than the - i/p (which is a steady 5.2V). If the engine gets so hot the diode pulls the + i/p below 5.2V then the LED turns on, but the o/p is held at 5.5V by the zener. The capacitor charges enough that the + i/p goes higher than 5.2V again, and the LED goes off.

When the coolant level goes low, the control relay sends short -ve pulses to the + i/p, which is enough to trigger the flashing, but hardly affects the high hysteresis of the bimetallic strip.

^^^^ or something like that, assuming the circuit above is correct.
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Jim Crowther
1984 1.9l EJ22 Westy Wolfsburg Edition
Vespa GTS 300

[NorthOf50]

Nearly perfect Jim. The only alteration I can tell required (and it really makes little difference) is that the "control box" and the "ntc sensor" both enter/exit the gauge on the same wire, connection 14/6 on the 14 pin (at least for my '84) so technically there are only 3 connections to the gauge.

Great work, I hope this gets pinned and saved someone important. This will also help me track down my bad connections, which I think is a poor soldering job @ the 750k and 150k resistor.

Kudos for your hard work on getting this done.
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'84 1.9WBX Camper (Penny)
'88 2.1WBX Camper (Martin)

[fxr]

[crazyvwvanman]

Please note that you also drew 2 ground paths to the dashed line enclosed area when there is only 1.

I realize that it doesn't change the function but since the physical gauge has only 3 external connections it is less confusing to show only 3.

Mark

[fxr]

[NorthOf50]

Ok, finally I have some free time (and energy *children*)...

I suspected that I had a bad solder join on my board so I reheated them all and let the solder set again. Now my led lights up, hurray! It doesn't blink *sigh* but it is blue and very bright, that coupled with the GW audible siren I think might be "good enough" for now.

I'm not sure why it doesn't blink and I do have enough bits to rebuild it entirely a second time but I will put that lower down on the project list now.

Here's a video in case anyone is interested. I'm a fan of the brighter LED.

https://youtu.be/CvUIkT1ndAA

Thanks again to all who collaborated on this.
_________________
'84 1.9WBX Camper (Penny)
'88 2.1WBX Camper (Martin)

[crazyvwvanman]

I have not verified this myself but someone who seemed in a position to know once told me that the led itself was a flashing led version.

There are such things available today. Here is a link to examples but I don't know if any of these are suitable or not.

https://www.newark.com/c/optoelectronics-displays/led-products/flashing-blinking-leds#

Mark

[kirtfitzpatrick]

[fxr]

They didn't have flashing LEDs back then. They didn't even have blue ones!

>>>> The flashing initiating pulses come from the relay. <<<< Only partially correct, depending on year and coolant level function.
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Jim Crowther
1984 1.9l EJ22 Westy Wolfsburg Edition
Vespa GTS 300

[MarkWard]

[kirtfitzpatrick]

[crazyvwvanman]

The flashing absolutely doesn't come from the relay.
The flashing occurs at key on even with the relay removed from the van.

With a bare gauge in my hand I can set the +V and Ground studs on the contacts of a 9 volt battery.
The led will immediately start flashing.

Mark